Market requirements are driving the need for a mud motor design that may build high doglegs yet also be rotated rapidly from the surface in order to maximize a rate of geological formation penetration such that boreholes may be drilled to a target depth in as short a time as possible. Such an assembly should also be reliable as well as be able to efficiently drill vertical, high dog leg severity curves and lateral sections in one run.
Present drillstrings typically use short bit-to-bend motors. However, these motors have limitations on maximum surface string revolutions per minute (RPM). These string RPM limitations may have a negative impact on rate of penetration (ROP) performance, especially in a lateral section.
Present drillstrings may also use an external bent housing. However, mud motors with an external bent housing may have endurance problems in the threads and upsets between a bearing pack and a power section. Bend limits for speed are traded against each other in order to maintain some semblance of fatigue management based on historical failure experience.
In short, there are general needs for a mud motor configuration that provides high surface rotation speed in vertical and tangent/lateral directions while providing improved fatigue life expectations.